It is conventionally known that a method for measuring the rotation angle of a rotating moving shaft of a machine tool has steps including, for example, fastening a polygon mirror to the rotating moving shaft of the spindle of a lathe, the rotary table of a machining center or the like, indexing the rotating moving shaft every angle corresponding to a division angle of the polygon mirror, and measuring the deflection of light reflected from a reflection member that points in a fixed direction with an autocollimator or the like (hereinafter referred to as a “method using a polygon mirror”).
In the method using a polygon mirror, measurable angles are determined by the division angle of the polygon mirror. For example, a 6-faceted mirror and an 8-faceted mirror permit measurement at every 60 degrees and every 45 degrees, respectively. To more minutely set the angle, the number of faces needs to be increased. This disadvantageously increases the cost of the polygon mirror, thus implementation is difficult. Such a method using a polygon mirror has a disadvantage in setting it minutely although the method offers relatively simple measurement.
To cope with the disadvantage, a method using a Hirth coupling is proposed, as is disclosed for example in PTL 1. This method includes the steps of rotating the rotating moving shaft of a machine tool fitted with a Hirth coupling by a predetermined unit angle each time, rotating the Hirth coupling backward by a unit angle each time after the Hirth coupling is disconnected, and repeatedly rotating the Hirth coupling in forward and reverse directions until the total of the rotation angles measured with a laser interferometer reaches a target displacement angle of the rotating moving shaft.